Various different loop links exist having ring topology and whose operating and connection characteristics are defined in the standard IEEE 802.
Within this document, standard 802-5 is applicable to ring topologies using a token access method, and it defines level 1 (physical level) and a portion of level 2 referred to as Medium Access Control (MAC), i.e. the lower layer of level 2. This is the data link level for defining the control of access to the ring.
Standard 802-2 relates to defining logic link control (LLC), i.e. a different portion of level 2, referred to as the upper layer.
Standard 802-1 relates to higher levels.
On the ring, a pattern of several bytes, called a token, circulates permanently. If no station is transmitting, then the token is free. When a station A desires to transmit, it seizes the token, marks it as being occupied, and transmits data in the form of a message to a destination station B. No other station can then transmit. As the message goes past, station B recognizes its own address, copies the data intended for itself, and marks an acknowledgement. On return of the message, the transmitting station A recognizes the acknowledgement and deletes the transmitted data together with the token busy state. The token is thus freed for all stations.
Using standard 802-5 has the following advantages:
a large number of stations, up to 256, can be connected to the ring, and stations can easily be added without interrupting traffic;
links between stations are asynchronous;
a single station can broadcast simultaneously to a plurality of other stations or to all of them;
the quality of message transmission between stations is excellent; and
depending on requirements procedures are available for operating and protecting the ring.
With respect to quality of transmission and operating security, application of standard 802-5 makes the following possible:
automatic declaration of each station merely by being physically plugged onto the ring;
diagnosis of the access interface at each connection;
automatic declaration of a station having the task of token surveillance, and automatic replacement thereof by a different station in the event of the first station failing; and
reconfiguration of the ring in the event of failure.
The characteristics of a token ring are the following:
At the physical level:
asynchronous point-to-point transmission enabling long physical connections to be used;
choice of physical medium depending on performance requirements, e.g. screened telephone pair, coaxial cable, or optical fiber;
various data rates: 4 Mbit/s; 16 Mbit/s; 100 Mbit/s;
an encoding law (Manchester) providing a first level of transmission protection;
data protecion by means of a cyclic redundancy check for detecting errors; and
standardized components and protocols for the 4 Mbit/s version (standard 802-5);
At link level:
the lower layer MAC of level 2 is standardized (standard 802-5) and integrated in circuits of the adapters, with an example of such a circuit being sold by Texas Instruments under the reference TMS 380;
the main services provided by the MAC lower layer are:
variable message length;
integration of level 2 protection;
point-to-point dialog or broadcast; and
managing various different priority levels.
A disturbance on the ring may be of limited duration, as occurs, for example, when a station is inserted or withdrawn, or it may be of longer duration in the event of a breakdown. Mechanisms are provided at the MAC lower layer for protecting the ring, and indeed for completely eliminating the station responsible for a breakdown. If detection and confinement of a breakdown take several milliseconds to several seconds or even tens of seconds, that is not compatible with the requirements of data transmission operating in real time, as is the case, for example, when switching in a telecommunications exchange. The LLC upper layer of level 2 is generally implanted in each adapter and is not suitable for real time operation.
The object of the invention is to allow stations to interchange messages in real time by means of a token ring type loop and to allow them to continue by means of a token ring type loop and to allow them to continue interchanging messages during a disturbance of said loop.
Another object of the invention is to avoid losing any messages during a disturbance and to avoid messages getting out of sequence.